The present invention relates to a numerical control apparatus and, particularly, to such apparatus for a composite machine tool having a rotary axia and a pair of linear axes perpendicular to each other and to the rotary axis, which is capable of simultaneously interpolating the three axes according to a displacement instruction in a rectangular coordinate system which is easily programmable.
In a machine tool such as lathe, a workpiece is machined into a body of revolution and, therefore, a machining operation is controlled in a cylindrical coordinate system according to instructions programmed in a cylindrical coordinate system. When the workpiece is to be machined into a configuration other than a body of revolution, a milling machine having control axes in a rectangular coordinate system is utilized usually.
When the workpiece is to be milled by a lathe with an aid of a numerical control apparatus, it is necessary to transform displacement instructions for the linear axes in the rectangular coordinate system into displacement instructions for a rotary axis and the linear axes in the cylindrical coordinate system, which requires very complicated programming. Since it is usual that such transformation has been performed manually, the number of samplings is limited, causing a machining accuracy to be degraded comparing with a machine tool having control axes in the rectangular coordinate system.